PII: S0038-092X(98)00036-X
Solar Energy Vol. 64, Nos 1–3, pp. 87–97, 1998
Published by Elsevier Science Ltd
Printed in Great Britain
0038-092X/98 $—see front matter
WATER DISINFECTION FOR DEVELOPING COUNTRIES AND
POTENTIAL FOR SOLAR THERMAL PASTEURIZATION
JAY D. BURCH and KAREN E. THOMAS
National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO 80401, U.S.A.
Received 25 September 1997; revised version accepted 25 February 1998
Communicated by JAMES BOLTON
Abstract—Water-borne disease in developing countries leads to millions of deaths and billions of illnesses
annually. Water disinfection is one of several interventions that can improve public health, especially if
part of a broad program that considers all disease transmission routes and sustainably involves the
community. Considering water volumes =30 m3/day, appropriate disinfection methods include chlorination, slow sand filtration, ultraviolet ( UV ) radiation and pasteurization. Pretreatment with a coarse
roughing filtration is generally used with the first three of these technologies to reduce turbidity and
maintain high effectiveness. Cysts and worm eggs are resistant to chlorination and UV but can be filtered
relatively easily. Chlorination is widely used and inexpensive but requires a continual supply of chemicals.
Slow sand filtration is lowest in cost but requires high investment in labor. Household filtration using
indigenous devices requires little capital investment but is relatively ineffective and difficult to properly
maintain. Batch treatment with solar UV is very easy to implement but effectiveness in practice is uncertain
since temperatures above 50°C should be attained. UV lamp devices are inexpensive and easy to use but
require power and access to maintenance infrastructure. Boiling of water requires no initial expense but
fuel and labor costs are very high. Solar pasteurization devices (batch and flow-through) are effective and
relatively maintenance-free, but existing products yield high treatment cost. Flow-through systems with
selective flat plate collectors become cost-competitive with UV technology at costs of about $380/m2 and
$80/m2 for home-scale and village-scale use, respectively. These cost goals might be attained with polymer
thin film designs if durability issues are adequately resolved. Published by Elsevier Science Ltd.
water characteristics. Water sources, in order of
decreasing quality, include springs, boreholes,
sealed wells, hand-dug wells, streams, rivers,
and lakes. Springs and boreholes tap groundwater sources that have been filtered through
layers of soil and rock and are isolated from
the surface. These sources may contain unpleasant color, odor, or minerals, but generally are
free from pathogen contamination and should
not require disinfection. Sealed wells are shallow
wells that have been sealed with cement around
a pump to prevent contamination. However,
contamination is possible, and sealed wells are
often treated with chlorine. Wells become contaminated from improper usage and from contaminated water entering the well from above,
particularly during flooding. Hand-dug wells
are typically contaminated. Finally, streams,
rivers, and lakes usually contain pathogens and
require disinfection.
1. INTRODUCTION
1.1. Problem characterization
The need for water disinfection in the developing world is indisputable. Water-borne diseases cause about five million deaths per year,
at least half of which are children ( UNICEF,
1995). The average child experiences more than
two episodes of diarrhoea each year; frequent
episodes of diarrhoea leave victims weakened
and malnourished, resulting in greater susceptibility to other diseases and, for adults, loss of
productivity (Snyder and Merson, 1982).
Roughly 50% of hospitalizations are from
water-borne disease (Alward et al., 1994). The
major pathogens of concern, their sizes, and
associated diseases are listed in Table 1. The
fecal–oral cycle is the basis of transmission of
most of these pathogens.
The source of the water largely determines
Table 1. Water-borne pathogens
Pathogen class
Bacteria
Viruses
Protozoa
Helminths (worms)
Size
Diseases
0.5–2 mm
20–80 nm
4–20 mm (cysts)
0.03–2 mm (eggs)
Diarrhoea, cholera, enteric and typhoid fever, dysentry
Heptatis, polio, diarrhoea, meningitis, lung diseases
Giardiasis, amoebic dysentery, diarrhoea
Round worm, guinea worm, schistosomiasis
87
88
J. D. Burch and K. E. Thomas
Water disinfection significantly improves
public health, but in and of itself is not a
panacea for water-borne diseases. There are
other means of transmission. Any contact
between the feces of a contaminated person and
what another person ingests (water, food, utensils, dirt) may result in spread of disease.
Education of the users is crucial so that they
do not dip unwashed hands or utensils into
storage vessels, especially when no residual disinfectant is present. Other intervention measures
are also needed, including increased supply and
sanitation. Increased water supply leads to
better hygiene. Widespread effective waste sanitation breaks the fecal–oral cycle at the source
and reduces the pathogen intake. Generally, the
balance between intervention measures (disinfection, hygiene education, additional water
supply, and sanitation) should be carefully
weighed and optimized ( Feachem et al., 1977).
Thus, it is important to be realistic about the
benefits that will actually be obtained from
water disinfection. Observed morbidity reductions for single and combined measures are
shown in Table 2. Apparent inconsistencies in
Table 2 reflect the high variability in the limited
data sets available.
1.2. Water disinfection: general issues
Just listing some of the relevant technical and
sociological variables related to water disinfection helps in understanding why the problem
has remained so pervasive for so long. Relevant
variables include: water pathogens—bacteria
and viruses are commonly found in nearly all
surface water and most groundwater, but protozoa and worms are less widespread, require
different treatment, and often have similar
symptoms; water turbidity—clean well water to
turbid river water (affects filtration design in
multi-stage treatment); local population density—urban, village, and widely dispersed single
family (impacts disinfection system capacity);
water use—from several to several hundred
Table 2. Observed reductions in diarrhoeal disease morbidity ([Esrey et al., 1991])
Improvement
Hygiene
Water quantity
Sanitation
Water quality
Water quantity and sanitation
Water quality and quantity
Mean reduction in
morbidity (%)
33
27
22
17
20
16
liters per day per person (depends heavily on
water supply method ); availability of electricity—reliable, questionable, or none (impacts
effectiveness and/or cost of systems that require
electricity); existing water disinfection—acceptable, questionable (some urban dwellers boil
‘‘treated’’ water), or none; local labor cost—low
labor costs make labor-intensive technologies
more attractive; community structure—complex
cultural issues can be barriers to functioning
water treatment management structures; infrastructure issues—varying access to supplies,
training for operation, maintenance and repair,
management support, and method of billing;
sanitation practices—affects exposure locally
and ‘‘downstream’’; hygiene practices—dependent on water supply and culture (other transmission paths); income—affects ability to pay
for (and thus sustain) water disinfection; and
awareness of disease (the fecal–oral cycle)—
affects motivation to invest in and maintain
water disinfection.
Many authors emphasize two points related
to these many complex variables. First, willingness to invest in water disinfection is crucial:
without community involvement and development of infrastructure (such as means of billing
to sustainably fund maintenance needs), any
water disinfection facility becomes nonfunctional and is useless ( World Bank, 1993).
Second, access to supplies, spare parts, and
training (all obviously important for facility
maintenance) heavily impacts technology
choice. A useful anecdote is the failure of wellsealing programs when an infrastructure to keep
the hand pumps operational was not also implemented ( WASH, 1993).
Water turbidity also impacts the choice of
water disinfection methods. Turbidity is a measure of the amount of solid particles suspended
in water, commonly determined by light scattering and measured in ‘‘nephelometric turbidity
units’’ (NTU ). The turbidity of well water is
quite low (<10 NTU ), while the turbidity of
dirty river and lake water varies widely
(10–2000 NTU ). Turbidity is caused by suspended material such as small particles (e.g. bits
of organic matter), fecal matter, or colloids
(micron-sized clay particles). These particles can
reflect or absorb ultraviolet ( UV ) radiation,
decreasing the effectiveness of UV disinfection.
In addition, these particles, particularly colloids,
serve as shelters for microorganisms, shielding
them from UV and chemical disinfectants.
Finally, high turbidity levels cause filters to
Water disinfection for developing countries and potential for solar thermal pasteurization
become clogged rapidly, thereby increasing the
maintenance needs of filters. Each technology
(except for pasteurization devices) specifies maximum turbidity for inlet water.
The effectiveness of disinfection systems also
depends on the types of pathogens present.
Protozoa form cysts when under stress. Cysts
have a tough, protective encapsulation that is
resistant to UV and chemical disinfection.
Worms and worm eggs are also resistant to UV
and chemical disinfection. Viruses are difficult
to remove by filtration because of their small
size (slow sand filters are an exception). Some
bacteria contain enzymes that repair their DNA
after damage by UV radiation, in presence
of light (photoreactivation) or without light
( Wegelin et al., 1994; Ellis, 1991). These mechanisms allow bacteria to gradually ‘‘regenerate’’
themselves after UV exposure. Wegelin et al.
(1994) show regeneration data with significant
repair after as little as 24 h. Therefore, water
treated by UV should be used within a reasonable time after disinfection, with one recommendation being within 36 h (Gadgil and Shown,
1997).
An important concern with any water disinfection that does not leave residual disinfectant
in the water is the potential for recontamination
after disinfection and before ingestion.
Recontamination often occurs from poor
hygiene and storage container handling. Leaky
distribution pipes can be contaminated during
periods of heavy rains. Of the treatments discussed here, only chlorination leaves a residual
in water that can disinfect subsequent contamination. Disinfection of water just before it
is used removes some of the potential for
recontamination.
Classes of water disinfection methods include
chlorination, filtration, UV, and pasteurization.
This paper discusses and compares specific
methods appropriate for smaller volume applications (=30 m3/day) in developing countries.
Technologies are selected on the basis of low
cost (actual or potential ) and/or historical use
by various water development programs.
2. APPROPRIATE WATER DISINFECTION
ALTERNATIVES
Technologies are evaluated on the basis
of normalized costs and appropriateness.
Appropriateness indicators include technology
effectiveness (including residual disinfection),
and technology ‘‘convenience’’ (relating to need
89
for supplies, and high and low cost labor). The
normalized cost indices are the cost of treated
water (C
) and the capacity cost (C ), based
water
cap
on standard life-cycle costs (LCC ), including
first cost and all operation and maintenance
costs. These costs are computed using the present worth factor (PWF ) and diurnal volumetric capacity (V ):
d
LCC=C +C ×PWF(N
, d, i )
0
OM
years
C
=LCC/[PWF(N
, d, i )×V ×365]
water
years
d
C =C /V
cap
0 d
The first cost C includes hardware cost and
0
installation labor; if hardware is imported, the
FOB (‘‘freight on board’’) cost is multiplied by
1.3 to account for international business costs.
Grid electricity is assumed as 10 ¢/kWh (if
applicable). C
is the cost for annual fuel,
OM
materials, repair and operating labor. d is the
discount rate (0.2 assumed ), i is the inflation
rate (0.1 assumed). N
is the system lifetime
years
in years. The technology lifetime is taken as
20 yr, except for solar systems (15 yr for metal
systems, 10 yr for polymer systems), home
ceramic filters (2 yr) and plastic bottles (1 yr).
Detailed assumptions for each technology are
given in Burch and Thomas (1998a) and results
are summarized in Table 3. It is emphasized
that these costs (especially with chlorination
and filtration) have large ranges and tend to
show considerable variation between references.
The costs reported here are at the low end of
costs purported to be applicable to developing
countries.
2.1. Chlorination
Chlorine is the most common form of water
disinfection used worldwide. Small-scale dosing
plants and household batch methods are considered here. Chlorine is relatively low-cost [costs
assumed are 1 ¢/g for batch use ( Water for
People, 1997), and 0.5 ¢/g for dosing plants]. A
primary advantage of chlorine is that the chlorine residual disinfects recontaminated water.
The chlorine residual depends on several factors
besides the added dose, including ‘‘biological
oxidation demand’’ and pH. The primary disadvantage of chlorine is that a constant supply is
needed, because liquid bleach degrades over
time (half-life on the order of two months,
depending upon whether the container is sealed
when not in use). Bleaching powder has a longer
half-life, on the order of one year, depending
on whether it is kept dry (Harris, 1992). Cholera
outbreaks have been reported in India when
24000
24000
7200
200
60
500
14
20
23
570
304
Production
l/day
2400
2160
2366
0
20
381
1
0
78
2145
84
First cost
$
100
90
329
0
333
761
43
0
3425
3764
276
Capacity cost
$/m3/day
6
3
14
9
85
63
133
2083
235
144
19
Life cycle cost
¢/m3
†Effectiveness scales: high=***, medium=**, low=*, none=blank (more stars=more effective).
‡Convenience scales: no need=***, low=**, medium=*, high=blank (more stars=more convenient).
¶Res=residual disinfection ability.
§B/v=effectiveness against bacteria and viruses.
#P/w=effectiveness against protozoa and worms.
††Sup=need for supplies.
‡‡Hi=need for high-skilled labor.
¶¶Lo=need for low-skilled labor.
§§PV=photovoltaics.
##Sol-UV=solar UV.
Chlorine dose+roughing filter
Slow sand+roughing filter
UV+PV (8 h)+filter
Chlorine batch
Household filter
Home UV+PV§§+filter
Sol-UV##/batch bottles
Water boiling (purchased fuel )
Batch solar: existing 1 m2
Flow-thru solar: existing 3 m2
Solar: potential polymer 1 m2
Variables/technologies
Units or subcategories
**
***
Effectiveness†
Res¶
***
**
***
***
*
***
*
***
***
***
***
B/v§
Table 3. Appropriate disinfection technologies: cost and appropriateness summary
***
***
***
***
**
***
**
*
**
**
P/w#
***
***
***
***
*
***
***
*
Convenience‡
Sup††
*
***
*
***
***
*
***
***
**
***
***
Hi‡‡
*
*
**
**
**
*
**
**
***
**
**
Lo¶¶
90
J. D. Burch and K. E. Thomas
Water disinfection for developing countries and potential for solar thermal pasteurization
impassable roads blocked the chlorine supply
during heavy storms (Gadgil and Shown, 1997).
A proper chlorine dose is complex to determine. The required chlorine base is specified in
parts per million (ppm) residual (mg/l ) times
exposure minutes (mg min/l ). The dose depends
on many factors. Higher doses are needed
for cysts and eggs (2–100 mg/min/l ) than
for viruses and bacteria (0.04–3 mg/min/l )
(Feachem et al., 1983; World Health
Organization, 1996). The dose increases roughly
eight-fold for an increase in turbidity from 1 to
10 NTU, increases roughly ten-fold for an
increase in pH from 6 to 10, and decreases
roughly ten-fold for a 20°C increase in water
temperature. Too high a chlorine dose makes
taste objectionable. Automated dosing plants
using chlorine gas, chlorine dioxide, and chloramines are suitable only for larger towns with
trained operators and accessible repair infrastructure. Bleaching powder is generally used
in developing countries because it is easier to
transport and handle safely. It may be applied
as a liquid solution in a drip-chlorination
plant. Chlorine dosing plants are costed at
$100/m3/day (Schulz and Okun, 1984), and an
average chlorine dose of 1 ppm is assumed. For
small-scale batch use, a 1% liquid solution may
be made from bleaching powder at a central
health post and distributed to individual households, which then add a given amount of the
solution to every bucket of water ( Water for
People, 1997).
2.2. Filtration
2.2.1. Roughing filter. To operate correctly,
chlorine, slow sand filtration, and UV disinfection systems require pretreatment of water with
high turbidity levels. For small-scale devices, a
‘‘roughing filter’’ is used, which is a multi-stage
filter with relatively coarse filtration media.
There are several possible configurations of
roughing filters for village-scale use, including
downward flow, upward flow, and horizontal
flow. Figure 1 shows a horizontal flow roughing
filter. The filter consists of 5 to 9 m total thickness of gravel in three layers: coarse gravel, fine
gravel, and coarse sand. The grain size and flow
rates are much larger than those used in slow
sand filters. The filter can remove up to
900 NTU of turbidity, and also removes about
90% of bacteria, protozoa, and worms ( Wegelin
et al., 1991). Operation is fairly straightforward,
requiring untrained laborers for most tasks.
Supplies of chemicals or spare parts are not
91
required. Head loss is normally less than 30 cm.
Maintenance consists of monthly cleaning by
rapidly flowing water through the filter.
Occasionally, manual removal, washing, and
replacement of filter media is required ( Wegelin
et al., 1991). A first cost for the roughing filter
is taken as $40/m3/day ( Water for People,
1997).
2.2.2. Slow sand filter. Slow sand filtration
(SSF ) is a very popular method of water disinfection among nongovernmental organizations
(NGOs) involved with water, as well as among
small towns in the developed world ( EPA,
1991). Figure 2 shows a schematic of a low
sand filter. Designs are available that mostly
eliminate the need for costly hardware (such as
valves and pumps). SSF involves filtering water
through about 100 cm of fine sand at a rate
slow enough for a biological film (the schmutzdecke) to develop on top of the sand. At the
slowest rates, water flows through the SSF at
about 2 m3/day/m2 surface area. This rate determines the SSF surface area. The top film serves
as a biological filter that effectively removes
over 99% of all pathogens (Schulz and Okun,
1984). SSF operation is fairly straightforward,
requiring untrained laborers for most tasks.
Supplies of chemicals or spare parts are not
required, which is a primary advantage of SSF.
The only energy requirement is for pumping to
compensate for 6 to 120 cm of head loss (head
loss increases gradually after scraping as the
biological layer builds). The majority of SSF
are designed for gravity-feed operation (Schulz
and Okun, 1984). Maintenance involves periodic raking of the biological film (every few
weeks, depending on water turbidity), scraping
off the top layer of sand every few rakings, and
replacement of a few inches of sand every few
scrapings. A disadvantage of SSF is that the
biological filter requires a period of several days
to ‘‘ripen’’ after every scraping. During the
ripening period, the filter does not effectively
remove bacteria. Therefore, multiple filters are
generally used in parallel. To reduce the frequency of scrapings, it is recommended that a
roughing filter be used for pretreatment of
waters of turbidity greater than about 20 NTU
( Water for People, 1997). The SSF first cost is
taken as $50/m3/day ( Water for People, 1997).
2.2.3. Household filtration. Household filters
common in the western world require periodic
media replacement and are far too expensive
for use in developing countries. Many indigenous designs are used, ranging from simple pots
92
J. D. Burch and K. E. Thomas
Fig. 1. A schematic of a horizontal roughing filter (from [ Wegelin et al., 1991]). Three stages are shown, water traveling
left to right through coarse gravel, finer gravel, and coarse sand. Viruses and bacteria will pass through the roughing filter.
filled with sand through which water is poured
to complex designs incorporating upward flow,
several ceramic layers, and multiple sand and
charcoal layers. One design is shown in Fig. 3.
A primary advantage of home filters is that
they can be produced by local craftspeople.
These filters form part of the traditional way of
life in many parts of the world. Testing of some
indigenous filters has shown that most can
remove 90–99.99% of bacteria and the finer
filters can also remove 90% of viruses (Gupta
and Chaudhuri, 1992). Note that an infective
dose for virus is a small number, and 90%
filtration may be of little value. Filter effectiveness is determined by the size of the filter
pores, absorptive forces within the filter media,
and the presence of cracks in the filter. Poorer
quality filters remove only 90% of bacteria,
which can mean that millions of bacteria may
be left behind in very poor quality water.
Ceramic filters produced by hand may vary
widely in quality or contain cracks. Finally,
filter quality can deteriorate over time, unknown
to the user. Filters must be regularly cleaned,
either by boiling or backwashing, and periodically replaced. For cost analysis, filter cost is
assumed at $20, production at 60 l/day, and
replacement at two years.
2.3. UV radiation
UV water disinfection (including technical
data, advantages, and disadvantages) is discussed by Schenck (1981) and Ellis (1991). UV
light from mercury discharge is produced near
250 nm, in the middle of the ‘‘germicidal band’’
from 200 nm to 280 nm. Germicidal UV radiation disables the DNA involved in reproduction
of bacteria and viruses, rendering these pathogens harmless upon ingestion. Lamp-driven UV
is a comparatively inexpensive and easy to use
technology. It has a relatively low power
demand. Systems can be designed for household
Fig. 2. A schematic slow sand filter (from [Feachem et al., 1977]). The water is filtered through fine sand, building up a
biological layer atop the sand (the ‘‘schmutzdecke’’). The sand is atop gravel and under drainage. Gravity feed is shown.
Water disinfection for developing countries and potential for solar thermal pasteurization
93
Fig. 3. A schematic home filter (from [Gupta and Chaudhuri, 1992]). An upflow unit is shown, with head provided by an
upper jar that feeds the lower filtration jar. The unit incorporates two sand layers separated from a charcoal layer by thin
cloth screens, and a gravel layer and a rock layer at the inlet for rough filtering.
or village-scale use. Photovoltaics (PV ) or other
renewable power sources are needed for offgrid locations. Filtering for cysts/worms and
lowering of turbidity to acceptable levels (=
20 NTU ) will frequently be needed, and is
assumed here. The roughing filter capital cost
is assumed to be $40/m3/day. A recent villagescale unit discussed in Gadgil and Shown (1997)
is projected to cost about US$525 FOB, and
produces about 21 m3 of water per day when
running continuously at maximum throughput
of 15 l/min. The 36 W UV bulb must be replaced
at about 8000 h of operation, and the ballast
lifetime is about 24,000 h. For off-grid locations
of interest here, PV is assumed added. The PV
system costs (including batteries and controller)
are taken as US$10/W FOB, with maintenance
costs of battery replacement plus 1% per year
of system first cost. System efficiency is assumed
to be 0.6. At an average daily irradiation of
5 kWh/m2, the cost of PV electricity is about
$1/kWh. With these assumptions, UV treatment
cost with (without) PV is 14 ¢/m3 (4 ¢/m3).
A home-scale UV+PV+filter unit is appropriate. An 8 W bulb/chamber combination is
available at very low cost (~US$50/unit FOB).
This unit is intended to run continuously.
Potentially, such a unit could be combined with
an appropriate filtration unit (not the multistage filters designed for developed world markets), though our study did not locate such
units commercially. An appropriate home filter
is estimated at $80/m3/day, which is twice the
cost of large-scale pretreatment filtering. When
combined with PV, such units appear a costeffective and convenient choice for single family
markets if access to technical infrastructure is
not an issue.
Because of basic black body effects and atmospheric absorption, natural UV from the sun is
relatively weak and predominantly at relatively
long wavelengths (>300 nm). It is much less
effective per unit energy than germicidal UV in
disabling pathogens, making its use challenging
at best. However, combining natural UV radiation and heat may be practical because of
synergistic heat/UV affects ( Wegelin et al.,
1994). Based on this mechanism, small-scale
batch techniques using appropriately blackened
plastic bottles and bags are being field tested
( Wegelin and Sommer, 1996). It is not clear
that sufficiently high temperatures will be
attained under cold and/or windy conditions.
However, being extremely simple, this approach
94
J. D. Burch and K. E. Thomas
appears useful if no other means of disinfection
is available. Cost analysis was done for plastic
bottles assumed to cost $0.01 and to last one
year.
2.4. Pasteurization
Thermal disinfection of liquids (water, milk,
etc.) is termed ‘‘pasteurization’’ after L. Pasteur
who first articulated the fundamental germ basis
of infectious diseases. Pasteurization by boiling
of water has long been recognized as a safe
way of treating water contaminated with enteric
pathogens. In fact, pasteurization can take place
at much lower temperatures than boiling,
depending on the time the water is held at the
pasteurization temperature. Process requirements for major pathogens are given in Feachem
et al. (1983) and are summarized in Fig. 4.
Pasteurization time decreases exponentially with
increasing temperature. Above 50°C, time
decreases at roughly a factor of 10 for every
10°C increase in pasteurization temperature.
Viruses appear the hardest to kill and essentially
set the boundary for acceptable time–temperature processes ( Feachem et al., 1983). A typical
process is 75°C for 10 min. The major advantage
of pasteurization is that apparently all major
pathogens of concern are killed, independent of
turbidity, pH, and other parameters influencing
alternative methods. Filtering is not required.
The major disadvantage of pasteurization is its
high cost.
2.4.1. Batch processes. Boiling water on a
home scale requires no initial capital and is very
effective, but has a very high cost in labor and
fuel. Gathered fuel may be free, but causes high
environmental damage and consumes inordinate amounts of time. Purchased fuel is assumed
for cost analysis here. The cost of boiling
water is taken as $0.02/l (Andreatta et al.,
1994), consistent with a charcoal cost of
$0.018/MJ and a stove efficiency of 25%. One
supplier (Hartzell, 1997) manufactures an efficient burner combined with heat exchanger to
preheat incoming water that increases the
throughput per unit fuel by 5–10 times, depending on the base case. Alternatively, solar heat
can be used. Andreatta et al. (1994) detail a
site-built batch system, which uses polymer thin
films and indigenous materials. This is a good
example of a potentially inexpensive system,
although the durability of the thin films needs
further investigation. Manufactured systems
include polybutylene tubes in a flat plate configuration costing US$60 FOB (Hartzell, 1997)
and an evacuated tube device costing US$95
FOB ( Hamasaki, 1997). Both devices have
about 20 l capacity and might cycle twice per
day under ideal circumstances. Although manufactured batch solar devices are relatively inexpensive and extremely easy to use, the devices
do not have large throughput, leading to relatively high treatment cost.
2.4.2. Flow-through solar thermal. Flowthrough solar thermal devices are discussed in
Andreatta et al. (1994). A schematic of a flowthrough solar thermal water pasteurization
system is shown in Fig. 5. Cold water entering
the system passes through the heat exchanger
first, where it is preheated by the hot pasteurized
water leaving the collector. The throughput of
the solar thermal devices of this type depends
primarily upon the collector parameters and the
effectiveness of the heat exchanger. After an
initial transient, the solar collector serves primarily to provide the heat needed because of
the ‘‘ineffectiveness’’ of the heat exchanger. A
detailed performance analysis can be found in
Burch and Thomas (1998b). Throughput is
more than 300 l/m2/(clear day) for flat-plate
collectors with selective absorbers. Maintenance
needs include rebuilding the control valve at
intervals depending on water characteristics.
Two systems are reported here: a recently-emergent 3 m2 system with a shell-in-tube heat
exchanger, costing US$1500 FOB and lasting
15 yr; and a potential 1 m2 system using thin
film polymers for collector and heat exchanger,
costing $64 and lasting for 10 yr. For cost
analysis, throughput must accommodate cloudy
conditions. Manufacturer’s data is used for the
existing system; an average value of 200 l/
m2/day is taken for the potential system.
It is important to note that solar devices with
metallic passageways can fail from freeze
damage and scale deposition. Freeze damage is
usually serious, amounting to ruptured piping,
and leads one to restrict the application of solar
devices with metal tubes to non-freezing climates. Scale deposition requires periodic descaling, such as with acetic acid. Good tools exist
for assessment of the potential severity of the
scaling problem, if data on relevant water characteristics are available ( Vliet, 1997). Caution
should be used in deploying any pasteurization
device with metal tubes in hard-water areas.
3. TECHNOLOGY COMPARISONS
An economic comparison of selected technologies is given in Table 3 and Fig. 6. In general,
Water disinfection for developing countries and potential for solar thermal pasteurization
95
Fig. 4. Temperature–time relationships for safe water pasteurization (from [Feachem et al., 1983]). The temperature (°C )
is on the vertical axis, and the time (h) is on the horizontal logarithmic axis. The hatched area in the upper right is the
‘‘safe zone’’ for all common pathogens. The lines represent safe zones for various specific pathogens.
comparison is confounded by wide cost ranges
for some technologies and by the complexity of
qualitative factors such as maintainability. For
village-scale, slow sand filtration is most cost
effective but requires significant low-cost labor.
Chlorination dosing plants and UV are also
relatively inexpensive village-scale approaches,
both requiring pretreatment to reduce turbidity
and filter cysts and worm eggs. Chlorination
requires a continual supply of chlorine and
trained operators. UV hardware is very simple
and quick to implement but requires access to
technical infrastructure for parts and maintenance. For home-scale, boiling has no capacity
cost but has very high fuel and labor costs.
Home filtration is low in first cost and utilizes
indigenous materials and labor, but it is costly
because of frequent replacement and is subject
to failure from poor workmanship or unobserved cracks. Home filtration devices have an
especially large cost range. The potential
PV-powered UV unit with prefiltering would be
convenient for homes with piped supply because
it is an ‘‘on demand’’ system requiring no
additional storage containers; its treatment cost
is linearly dependent on assumed draw volume.
Solar pasteurization might be an option on
both the village scale and the household scale.
Household usage is the more likely market
because village-scale alternatives have much
lower treatment cost. Existing solar devices have
water disinfection costs that are an order of
96
J. D. Burch and K. E. Thomas
Fig. 5. A schematic for solar pasteurization. Heated water
enters the counterflow heat exchanger from the collector
outlet, exchanging heat with the cold incoming water. The
control valve regulates the flow so that the desired pasteurization temperature is reached. The collector capacitance is
designed to provide sufficient residence time at maximum
flow rate.
magnitude less than boiling. Solar thermal pasteurization with existing manufactured devices
costs more than the remaining alternatives but
is highly effective and lowest in maintenance
(assuming for metallic systems that freezing and
scaling issues are eliminated by proper site
analysis before deployment). By assuming a
unit area productivity (200 l/m2/day was used
here) and equating the life cycle disinfection
cost of solar pasteurization with the best competition, unit area cost goals can be derived (Burch
and Thomas, 1998a). For village-scale treatment, the treatment cost with slow sand filter
and with UV+PV+filter is about 3 ¢/m3 and
14 ¢/m3, respectively; the solar cost goal is
about $18/m2 and $84/m2, respectively. For
home-scale, the treatment cost with the potential UV+PV+filter system is about 63 ¢/m3;
the corresponding solar cost goal is about
$380/m2. Existing metallic hardware could possibly achieve the latter costs, with about a factor
of two reduction to reach the home-scale goal.
Potential thin film polymer systems (Burch and
Thomas, 1998a) could achieve the home-scale
goal, but could meet the UV village-scale goal
only with very low mark-ups for profit and
distribution. It is not conceivable that any solar
product could compete with village-scale slow
sand filtration. Polymer film durability is an
unresolved issue, although some research is
underway as in Burch (1997).
4. CONCLUSIONS
The water disinfection problem in developing
countries is large and complex. There are a
number of appropriate methods for water disinfection, each with advantages and disadvantages. On the village scale, slow sand filtration
Fig. 6. Appropriate technology cost comparison. The y axis is log of the normalized costs. The hatched bar is the capacity
10
cost in $/m3, which is the first cost divided by the daily output of the system. The solid bar is the normalized life-cycle
treatment cost in ¢/m3.
Water disinfection for developing countries and potential for solar thermal pasteurization
is widely recommended. Chlorine dosing plants
will continue to be used. UV treatment is reasonably competitive if access to technical infrastructure for maintenance is adequate. On the
household scale, chlorine will continue to be
one of the most popular options for its low cost
and residual disinfection. Home-scale filtration
(perhaps combined with UV ) holds promise of
being effective and inexpensive.
Solar thermal pasteurization is effective and
low in maintenance, but existing systems have
higher life-cycle costs than most of the competition. For flow-through solar pasteurization costs
to equal the chosen alternatives here, user cost
goals are $84/m2 (village scale) and $380/m2
(home scale).
Acknowledgements—This work was supported by the U.S.
Department of Energy, Office of Utility Technologies, Solar
Buildings and Heat Program. The support of Frank Wilkins
is gratefully acknowledged. Substantial assistance was provided by Ashok Gadgil for UV and Les Hamasaki and Will
Hartzel for solar pasteurization.
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